This invention relates to an externally pressurized gas bearing spindle and particularly to a bearing spindle suitable for high-speed rotation.
FIG. 1 shows an externally pressurized gas bearing spindle (hereinafter referred to as an air spindle). This air spindle has bearing sleeves 2, 2xe2x80x2 mounted in a housing 1. A motor bracket 4 is joined to the rear end of the housing 1. A main shaft 5 is inserted into the bearing sleeves 2, 2xe2x80x2. A small-diameter portion at the rear end of the main shaft 5 is coupled to a rotor 6 of a motor 3 in the motor bracket 4. The motor 3 has its stator 7 supported by the motor bracket 4. A rotary encoder 8 is mounted on the rear end of the main shaft 5, which protrudes outwardly out of the motor bracket 4. The encoder 8 detects the number of revolutions of the main shaft 5.
At a portion near the motor 3, the main shaft 5 is provided with a thrust plate 11 in a flange-like manner. The thrust plate 11 is disposed between the bearing sleeve 2 and another bearing sleeve 2xe2x80x2.
A gas-supply passage 12 for radial bearing is provided so as to extend in a radial direction through the housing 1 and the bearing sleeve 2. A plurality of narrow diametric nozzles 13 are arranged annularly in four rows in an inner or radial bearing surface of the bearing sleeve 2.
The gas-supply passage 12 communicates with the four rows of nozzles 13. By supplying pressurized gas through the gas-supply passage 12, radial bearings 14, 14xe2x80x2 for the main shaft 5 are formed at two places. Exhaust is made through an exhaust passage 15 extending radially and axially through the housing 1.
A gas-supply passage 16 for a thrust bearing is provided so as to extend through the housing 1 and the bearing sleeve 2xe2x80x2. A plurality of narrow axial nozzles 17 are arranged annularly in a thrust bearing surface of the bearing sleeve 2xe2x80x2. The gas-supply passage 16 communicates with the nozzles 17. By supplying pressurized gas through the gas-supply passage 16, a thrust bearing 18 for the thrust plate 11 of the main shaft is formed. Exhaust is made through an exhaust passage 19 extending through the bearing sleeve 2xe2x80x2 and the motor bracket 4.
The motor bracket 4 has a cooling gas-supply passage 20 to air-cool the motor 3.
The number of the nozzles 13 of each radial bearing 14 is four to six for each row. The distance between the rows, i.e. the axial distance S between the nozzles, is set at 40-60% of the bearing width W.
For an air spindle for medium to low speed, it is required to set the bearing stiffness as high as possible and to minimize the consumption of gas supplied. In order to meet these requirements, the optimum bearing clearance is set at several micrometers.
Specifically, as shown in FIG. 4, the bearing clearance was set at 5-8 xcexcm (see the curve for a conventional product) to maintain the bearing stiffness as high as 0.9-1.0 kgf/xcexcm. As will be apparent from FIG. 4, since the optimum range of bearing clearance is narrow in width and if the bearing clearance gets out of the range, the bearing stiffness decreases sharply, high machining accuracy is required for the bearing portions.
Also, heretofore, the ratio of the supply gas flow rate Q (liter/min) to the bearing stiffness H was set substantially at one (see the lines for conventional products in FIGS. 2 and 3).
When the air spindle set as described above is used at high-speed rotation, the bearing friction loss increases markedly because it increases in proportion to the square of the number of revolutions. Since the load on the motor 3 also increases in proportion to the bearing friction loss, the motor current increases. Due to these factors, the temperature of the air spindle increases markedly during high-speed rotation.
When the temperature of the air spindle increases, heat is conducted to surrounding parts, causing thermal expansion, thus deteriorating the accuracy of the device. Also, if it is used in a disk examination device, if the main shaft 5 of the air spindle extends due to thermal expansion, it is possible that examination becomes impossible because the measuring range of a measuring sensor or a magnetic head cannot follow the displacement of the main shaft during examination.
Thus, conventional air spindles could be used only at a speed of 20000 r/min or under.
An object of this invention is to provide an air spindle for high-speed rotation in which heat generation is suppressed during high-speed rotation without lowering the bearing stiffness to minimize the elongation with temperature rise and influence of elongation on surrounding devices.
According to this invention, there is provided an externally pressurized gas bearing spindle comprising a housing, a bearing sleeve mounted in the housing, a main shaft supported in the bearing sleeve, a gas supply passage extending through the bearing sleeve and the housing, a plurality of narrow nozzles provided in the bearing sleeve, a radial bearing formed by supplying gas to a bearing surface between the bearing sleeve and the main shaft, the ratio of the supply gas flow rate Q to the bearing stiffness H of the radial bearing being set at 2-10.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: